The present invention relates to the growth of large single crystals of semiconductor materials for electronic applications and in particular relates to the bulk growth of gallium nitride single crystals for use as a substrate and related elements in semiconductor devices.
Gallium nitride (GaN) and its related Group III nitrides have gained significant interest and commercial acceptance as semiconductor materials in the last decade. As well understood in this art, the term Group III nitrides refers to binary, ternary and tertiary compounds formed by one or more of the elements from Group III of the periodic table, particularly gallium (Ga), aluminum (Al) and indium (In).
Gallium nitride is a wide bandgap (3.36 eV at 300 K) semiconductor which provides it with the capability of handling power applications at an order of magnitude or more better than semiconductors such as silicon (1.12 eV) or gallium arsenide (1.42 eV). The bandgap also provides light emitting diodes (LEDs) formed in gallium nitride and the other Group III nitrides with the capability of emitting photons that are in the green, blue, violet, and ultraviolet (i.e. higher energy) portions of the visible and near-ultraviolet spectrum. In turn, when blue LEDs are combined with red and other lower energy spectrum color LEDs or with appropriate phosphorus, they provide the opportunity to create white light.
Gallium nitride and the related Group III nitrides are also direct emitters; i.e. an electron hole recombination event creates energy entirely in the form of a photon and without the concurrent production of vibrational energy (e.g., a phonon). The loss of energy to a phonon in indirect (even if wide bandgap) semiconductors such as silicon carbide, reduces the light-generating efficiency of the emission. Stated differently, energy is always conserved in such transitions, but the emission of energy is entirely as light in a direct emitter while only partially as light in an indirect emitter. Thus, generally speaking and all other factors being equal, gallium nitride and the other Group III nitrides are more efficient in terms of producing visible light than is silicon carbide.
Gallium nitride has additional attributes that make it extremely desirable as a semi-conductor material. First, it has a high electron mobility and saturation velocity giving it the capability of amplifying high frequency signals. Second, gallium nitride can accommodate high voltages without breakdown, which is also important for rectifying and amplifying applications. Gallium nitride can also operate (in appropriate devices) at frequencies potentially as high as 40 gigahertz (GHz), and has a power density at least six (6) times as great as silicon. For these and other reasons, gallium nitride is also expected to be a significant foundation for the semiconductor devices required or desired for newer generations of wireless technologies.
The growth and incorporation of epitaxial layers of gallium nitride and related Group III nitrides has matured sufficiently for GaN to appear in many commercial devices, particularly light emitting diodes. Nevertheless, commercially viable, larger or “bulk” single crystals of gallium nitride remain an elusive target. As a result, most gallium nitride or Group III nitride-based devices are formed on sapphire (Al2O3) or silicon carbide (SiC) substrates. Sapphire transparency over a wide frequency range and its physical strength. It cannot be conductively doped, however, and thus adjustments must be made to the device designs to incorporate its insulating character. In particular, the insulating properties of sapphire require that two electrodes to be formed on a single side of a device, thus demanding valuable extra chip space and size.
Silicon carbide (SiC) offers further advantages over sapphire as a substrate material for GaN and other Group III nitrides. SiC can be conductively doped and has a better lattice match (a lattice constant of 3.086 Å) with GaN (3.189 Å) than does sapphire (4.913 Å). A slight lattice mismatch still remains, however, between silicon carbide and gallium nitride. Such mismatches generate defects (dislocations) during growth, during temperature cycling (because of differences in thermal expansion) and during use. Such defects are already recognized as reducing the life of nitride-based lasers and light-emitting diodes, and have even more serious ramifications for high power devices that must be able to withstand much higher biases.
From the standpoint of eliminating crystal mismatch problems from devices, the best solution in almost all circumstances is to have a substrate with an identical or extremely close chemical and physical (particularly lattice) match to the materials used for the active portions of the device. Thus, the availability of silicon carbide substrates for silicon carbide based devices helped drive the rapid growth in silicon carbide technology that began in the late 1980's. Accordingly, in order to provide the theoretically best substrates for gallium nitride and Group III nitride based devices, a logical goal is to provide gallium nitride substrates for such devices so that they can be identically lattice matched with their active portions. For a number of reasons, however, gallium nitride has been difficult to form into bulk single crystals with a low enough defect density to provide worthwhile substrates for electronic devices. There are a number of difficulties in attempting to grow gallium nitride in bulk crystal, rather than epitaxial, fashion.
Gallium nitride does not grow easily from a melt because of its relatively high decomposition rate. In response, techniques have included attempts at ultra-high pressure solutions and sublimation, but these have tended to suffer from significant growth rate problems or high complexity. Other more successful techniques such as pendeo and lateral overgrowth techniques (e.g., U.S. published Application 20020022290) have improved the quality of available gallium nitride, but may proceed at relatively low growth rates. Stated differently, because devices formed in epitaxial layers are relatively small, the relatively slow growth rates (of admitted higher quality) of epitaxial layers are acceptable. Nevertheless, slower growth rates are less acceptable for commercial production of semiconductor substrates, which need to have a thickness on the order of 30 mils (about 0.76 millimeters) and diameters of at least two inches, with even larger diameters being preferred.
Processes such as vapor phase epitaxy (“VPE”) and chemical vapor deposition (“CVD”) can also drive up manufacturing costs because of their frequent use of halogen (particularly chlorine) compounds. The use of halogens greatly increases the necessity for chemically resistant materials in growth chambers and systems, and both molecular chlorine (Cl2) and chloride ion (Cl−1) can generate a number of undesired and disadvantageous side reactions. Vapor phase epitaxy is also very slow with nominal rates of between about one and two microns per hour, and “aggressive” growth rates being only between about five and ten microns per hour. Such rate tend to be commercially unacceptable for growing gallium nitride crystals in the bulk sizes (thickness and diameter) useful for device substrates.
The crystal melt techniques require both pressures approaching 100 atmospheres and the use of liquid gallium because nitrogen will not dissolve in gallium at atmospheric pressure. By way of comparison gallium arsenide (GaAs) can be produced at pressures of about five (5) atmospheres and gallium phosphide (GaP) can be produced at pressures of about 35 atmospheres.
Sublimation techniques also seem to fail to date in gallium nitride, with growth rates appearing to simply stop after the process has proceeded for a reasonable period of time.
More recent techniques include those in which GaN is first grown on a non-GaN substrate and then removed in bulk fashion; e.g. U.S. Pat. No. 5,679,152 or 6,407,409. Such techniques can be considered variations of epitaxial growth rather than bulk substrate growth.
Accordingly, the need continues to exist for techniques for producing high quality single crystal gallium nitride in amounts and at rates and at crystal sizes, that are suitable for substrate applications in semiconductor devices.